An automatic transmission for a motor vehicle generally includes a number of gear elements coupling an input and output shaft, and a number of torque-transmitting devices that are selectively engageable to thereby activate certain gear elements, thus establishing a desired speed ratio between the input and output shafts. As used herein, the term “torque-transmitting device” will be used collectively to refer to brakes as well as rotating clutches.
The transmission input shaft is typically connected to a vehicle engine or another vehicle power source through a fluid coupling device such as a torque converter. A torque converter includes a turbine (the “driven” member), a pump (the “driving” member), and a stator (the “stationary” member), and the output shaft is connected directly to the vehicle wheels for propelling the vehicle. Shifting from one forward speed ratio to another is performed in response to engine throttle level and vehicle speed, and generally involves releasing or disengaging a clutch associated with the current speed ratio, i.e., the off-going clutch, and applying or engaging a clutch associated with a desired new speed ratio, i.e., the on-coming clutch.
The term “speed ratio” is defined herein as the transmission input speed or torque converter turbine speed divided by the transmission output speed. Thus, a low gear range has a high speed ratio while a high gear range has a lower speed ratio. A shift made from a high speed ratio to a lower speed ratio is referred to commonly as an “upshift”. In the type of transmission involved within the scope of this invention, an upshift is accomplished by disengaging a clutch associated with the higher speed ratio and engaging a clutch associated with the lower speed ratio to thereby reconfigure the gear set to operate at the lower speed ratio. Shifts performed in the above manner are termed “clutch-to-clutch” shifts, and require precise timing in order to achieve optimal quality shifting, and to reduce a perceptible delay in the upshift event.
Conventional transmissions typically use various compliance devices such as accumulators, wave plates, and orifices, as well as hydraulic line pressure, to control the shift event. The quality of a particular shift event, and in particular the minimization of a delay in executing an upshift, depends on cooperative operation of several different clutch functions, such as pressure changes within on-coming and off-going clutch apply chambers, and the timing of control of the various compliance devices.
In clutch-to-clutch systems, single clutches may perform multiple clutch functions. For example, one clutch may handle low torque, closed-throttle downshifts while remaining capable of handling a high torque upshift. Moreover, clutches are designed to meet requirements for durability, packaging, and different shifting scenarios. This balance in clutch-to-clutch systems may lead to less than optimal hardware utilization for certain clutch maneuvers. A high torque upshift may require rapid pressurization or fill of the clutch chamber while overcoming compliance to rapidly initiate a desired speed ratio change, which in some circumstances may lead to less than optimal upshift delays.